63,717 research outputs found
A least-squares implicit RBF-FD closest point method and applications to PDEs on moving surfaces
The closest point method (Ruuth and Merriman, J. Comput. Phys.
227(3):1943-1961, [2008]) is an embedding method developed to solve a variety
of partial differential equations (PDEs) on smooth surfaces, using a closest
point representation of the surface and standard Cartesian grid methods in the
embedding space. Recently, a closest point method with explicit time-stepping
was proposed that uses finite differences derived from radial basis functions
(RBF-FD). Here, we propose a least-squares implicit formulation of the closest
point method to impose the constant-along-normal extension of the solution on
the surface into the embedding space. Our proposed method is particularly
flexible with respect to the choice of the computational grid in the embedding
space. In particular, we may compute over a computational tube that contains
problematic nodes. This fact enables us to combine the proposed method with the
grid based particle method (Leung and Zhao, J. Comput. Phys. 228(8):2993-3024,
[2009]) to obtain a numerical method for approximating PDEs on moving surfaces.
We present a number of examples to illustrate the numerical convergence
properties of our proposed method. Experiments for advection-diffusion
equations and Cahn-Hilliard equations that are strongly coupled to the velocity
of the surface are also presented
Molecular dynamics in arbitrary geometries : parallel evaluation of pair forces
A new algorithm for calculating intermolecular pair forces in molecular dynamics (MD) simulations on a distributed parallel computer is presented. The arbitrary interacting cells algorithm (AICA) is designed to operate on geometrical domains defined by an unstructured, arbitrary polyhedral mesh that has been spatially decomposed into irregular portions for parallelisation. It is intended for nano scale fluid mechanics simulation by MD in complex geometries, and to provide the MD component of a hybrid MD/continuum simulation. The spatial relationship of the cells of the mesh is calculated at the start of the simulation and only the molecules contained in cells that have part of their surface closer than the cut-off radius of the intermolecular pair potential are required to interact. AICA has been implemented in the open source C++ code OpenFOAM, and its accuracy has been indirectly verified against a published MD code. The same system simulated in serial and in parallel on 12 and 32 processors gives the same results. Performance tests show that there is an optimal number of cells in a mesh for maximum speed of calculating intermolecular forces, and that having a large number of empty cells in the mesh does not add a significant computational overhead
Perspectives on the simulation of micro gas and nano liquid flows
Micro- and nano-scale fluid systems can behave very differently from their macro-scale counterparts. Remarkably, there is no sufficiently accurate, computationally efficient, and — most importantly — generally agreed fluid dynamic model that encapsulates all of this important behaviour. The only thing that researchers can agree on is that the conventional Navier-Stokes fluid equations are unable to capture the unique complexity of these often locally non-thermodynamic-equilibrium flows. Here, we outline recent work on developing and exploring new models for these flows, highlighting, in particular, slip flow as a quintessential non-equilibrium (or sub-continuum) phenomenon. We describe the successes and failures of various hydrodynamic and molecular models in capturing the non-equilibrium flow physics in current test applications in micro and nano engineering, including the aerodynamic drag of a sphere in a rarefied gas, and the flow of water along carbon nanotubes
Hydrodynamics of Micro-swimmers in Films
One of the principal mechanisms by which surfaces and interfaces affect
microbial life is by perturbing the hydrodynamic flows generated by swimming.
By summing a recursive series of image systems we derive a numerically
tractable approximation to the three-dimensional flow fields of a Stokeslet
(point force) within a viscous film between a parallel no-slip surface and
no-shear interface and, from this Green's function, we compute the flows
produced by a force- and torque-free micro-swimmer. We also extend the exact
solution of Liron & Mochon (1976) to the film geometry, which demonstrates that
the image series gives a satisfactory approximation to the swimmer flow fields
if the film is sufficiently thick compared to the swimmer size, and we derive
the swimmer flows in the thin-film limit. Concentrating on the thick film case,
we find that the dipole moment induces a bias towards swimmer accumulation at
the no-slip wall rather than the water-air interface, but that higher-order
multipole moments can oppose this. Based on the analytic predictions we propose
an experimental method to find the multipole coefficient that induces circular
swimming trajectories, allowing one to analytically determine the swimmer's
three-dimensional position under a microscope.Comment: 35 pages, 11 figures, 5 table
Analytical ray-tracing in planetary atmospheres
Ground-based astro-geodetic observations and atmospheric occultations, are
two examples of observational techniques requiring a scrutiny analysis of
atmospheric refraction. In both cases, the measured changes in observables are
geometrically related to changes in the photon path and the light time of the
received electromagnetic signal. In the context of geometrical optics, the
change in the physical properties of the signal are related to the refractive
profile of the crossed medium. Therefore, having a clear knowledge of how the
refractivity governs the photon path and the light time evolution is of prime
importance to clearly understand observational features. Analytical studies
usually focused on spherically symmetric atmospheres and only few aimed at
exploring the effect of the non-spherical symmetry on the observables. In this
paper, we analytically perform the integration of the photon path and the light
time of rays traveling across a planetary atmosphere. We do not restrict our
attention to spherically symmetric atmospheres and introduce a comprehensive
mathematical framework which allows to handle any kind of analytical studies in
the context of geometrical optics. To highlight the capabilities of this new
formalism, we carry out five realistic applications for which we derive
analytical solutions. The accuracy of the method of integration is assessed by
comparing our results to a numerical integration of the equations of
geometrical optics in the presence of a quadrupolar moment . This shows
that the analytical solution leads to the determination of the light time and
the refractive bending with relative errors at the level of one part in
and one part in , for typical values of the refractivity and the
parameter at levels of and , respectively
Dynamics and Topological Aspects of a Reconstructed Two-Dimensional Foam Time Series Using Potts Model on a Pinned Lattice
We discuss a method to reconstruct an approximate two-dimensional foam
structure from an incomplete image using the extended Potts mode with a pinned
lattice we introduced in a previous paper. The initial information consists of
the positions of the vertices only. We locate the centers of the bubbles using
the Euclidean distance-map construction and assign at each vertex position a
continuous pinning field with a potential falling off as . We nucleate a
bubble at each center using the extended Potts model and let the structure
evolve under the constraint of scaled target areas until the bubbles contact
each other. The target area constraint and pinning centers prevent further
coarsening. We then turn the area constraint off and let the edges relax to a
minimum energy configuration. The result is a reconstructed structure very
close to the simulation. We repeated this procedure for various stages of the
coarsening of the same simulated foam and investigated the simulation and
reconstruction dynamics, topology and area distribution, finding that they
agree to good accuracy.Comment: 31 pages, 20 Postscript figures Accepted in the Journal of
Computational Physic
"Regularity Singularities" and the Scattering of Gravity Waves in Approximate Locally Inertial Frames
It is an open question whether solutions of the Einstein-Euler equations are
smooth enough to admit locally inertial coordinates at points of shock wave
interaction, or whether "regularity singularities" can exist at such points.
The term {\it regularity singularity} was proposed by the authors as a point in
spacetime where the gravitational metric tensor is Lipschitz continuous
(), but no smoother, in any coordinate system of the atlas.
An existence theory for shock wave solutions in admitting arbitrary
interactions has been proven for the Einstein-Euler equations in spherically
symmetric spacetimes, but is the requisite smoothness required for
space-time to be locally flat. Thus the open problem of regularity
singularities is the problem as to whether locally inertial coordinate systems
exist at shock waves within the larger atlas. To clarify this open
problem, we identify new "Coriolis type" effects in the geometry of
shock wave metrics and prove they are essential in the sense that they can
never be made to vanish within the atlas of {\it smooth} coordinate
transformations, the atlas usually assumed in classical differential geometry.
Thus the problem of existence of regularity singularities is equivalent to the
question as to whether or not these Coriolis type effects are essentially
non-removable and `real', or merely coordinate effects that can be removed, (in
analogy to classical Coriolis forces), by going to the less regular atlas of
transformations. If essentially non-removable, it would argue
strongly for a `real' new physical effect for General Relativity, providing a
physical context to the open problem of regularity singularities.Comment: 29 pages. Version 2: Corrections of some typographical errors and
improvements of wording. Results are unchange
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